Show CDP Neighbors
This command lists the Cisco devices that are connected to your current device. CDP is a Cisco proprietary protocol and will only detect Cisco products, although there are some vendors that do work with it.
Additionally, LLDP was recently released, which is an industry standard of the Cisco Discovery Protocol and is supported on newer IOS.
Router#show cdp neighbors
In the below example we use show cdp neighbors to see what devices are directly connected to R2.
R2#show cdp neighbors
Device ID Local Intrfce Holdtme Capability Platform Port ID